As a conventional motor drive assembly for a vehicle used in electric vehicles and hybrid vehicles, a motor drive assembly is disclosed (e.g. in Japanese Unexamined Patent Application Publication No. 2011-58534) which includes an electric motor, a speed reducer which reduces the rotation speed output from the electric motor, and a differential which distributes the rotation output from the speed reducer to right and left vehicle wheels, the speed reducer having a plurality of interchangeable speed reduction ratios.
By use of this motor drive assembly, it is possible to use the optimum speed reduction ratio of the speed reducer according to the travel conditions, and thus to use the electric motor in the rotation range and torque range in which energy efficiency is high, both while the vehicle is being driven and during regenerative braking. Also, it is possible to reduce the rotation speeds of rotating members of the speed reducer by use of the optimum speed reduction ratio, while the vehicle is travelling at a high speed. As a result thereof, it is possible to reduce the power loss of the speed reducer and thus improve the energy efficiency of the vehicle.
In recent years, since the issue of global environment or energy is drawing a lot of attention, the development of electric vehicles or hybrid vehicles is being promoted accordingly. Now, it is recognized as an important issue to improve the energy efficiency of electric vehicles or hybrid vehicles, and thus it is becoming important to lighten the weight of the vehicles.
The inventor of the present invention studied how to lighten an electric motor, which is a driving source of a motor drive assembly for a vehicle, so as to lighten the motor drive assembly, and made an experiment to evaluate an electric motor configured as illustrated in FIG. 13.
An electric motor 90 illustrated in FIG. 13 includes an annular stator 91, and a rotor 92 arranged inside of the stator 91. The rotor 92 receives a rotating force from the stator 91 so as to rotate. The rotor 92 includes a spindle 94 supported by a pair of bearings 93 arranged so as to be axially spaced apart from each other, and a rotor core 95 mounted on the outer periphery of the spindle 94.
In order to lighten the spindle 94, the spindle 94 includes a tubular, rotor core mounting portion 96 which has its both ends open, a shaft 97 axially extending through the interior of the tubular, rotor core mounting portion 96, and a flange 98 through which the shaft 97 and the rotor core mounting portion 96 are coupled together. This spindle 94 is lightweight, since the rotor core mounting portion 96 is hollow even though the shaft 97 and the flange 98 are not hollow.
However, the inventor discovered that in the arrangement of the electric motor 90 illustrated in FIG. 13, since the shaft 97, of which the “second moment of area” is small, receives moment loads applied between the pair of bearings 93 supporting the spindle 94, torsional vibration or flexural vibration tends to occur in the shaft 97, which might cause vibration or abnormal noise to occur from the electric motor 90, while the electric motor 90 is rotating at a low speed.